Hitherto, in the application field of tires for which an abrasion resistance and a mechanical strength are required, there have been extensively used rubber compositions that are enhanced in mechanical strength by incorporating a filler such as carbon black or silica in a rubber component such as a natural rubber and a styrene-butadiene rubber. Since the rubber composition containing a filler has a high viscosity during the kneading, rolling and extrusion, a plasticizer such as a processed oil is used for the purpose of improving processability or flowability. Since such a plasticizer can soften the rubber composition after the molding process, the plasticizer also has a function as a softener of the rubber composition.
In the meantime, in the use as a tire described above, there is a problem that even if the rubber has suitable physical properties such as mechanical strength and hardness at the time of production, the performance of the rubber changes with time during the use for a long time. This is caused by the migration of the plasticizer from the inner side of the rubber to the outside. As a method for suppressing the change in the rubber performance due to the migration of the plasticizer, Patent Document 1, for example, discloses a rubber composition containing a petroleum hydrocarbon resin and a specific vulcanization accelerator; and Patent Document 2 discloses a rubber composition containing a diene liquid polymer, diene rubber component and a filler at a specific ratio.
However, the tires described in Patent Documents 1 and 2 do not simultaneously have a mechanical strength, a hardness, a rolling resistance performance and a processability at a sufficiently high level, so that further improvements are demanded.
As the methods for improving the processability of rubber compositions, a method in which carbon black having a large particle size is used, and a method in which the content of the carbon black is decreased are also known. However, these methods have a drawback in that the mechanical strength, abrasion resistance or hardness is deteriorated.
Thus, development of a material having a rolling resistance performance together with a low-migration performance of the plasticizer and so on is demanded.